The present invention relates to a measurement technique for ultra-thin oxides and, more particularly, to a technique that utilizes a monitor sample in conjunction with a device-to-be measured to provide for an accurate measurement of a deposited oxide film.
In the prior art, there has been considerable interest in developing methods and apparatus for measuring the thickness of thin films on substrates. This need is particularly acute in the semiconductor manufacturing industry where extremely thin films are deposited on silicon substrates.
One technique for measuring such films is defined broadly as xe2x80x9cellipsometryxe2x80x9d. In an ellipsometer, a probe beam, having a known polarization, is directed to reflect off the surface of the film being measured at an oblique angle. The thickness of the film layer on the substrate will affect the polarization state of the reflected beam. That is, there will be a change in relative phase shift between the xe2x80x9cPxe2x80x9d wave, which is a vibration component parallel to the plane of incidence, and the xe2x80x9cSxe2x80x9d wave, which is a vibration component perpendicular to the plane of incidence, as well as the change in amplitude ratio between the two waves. Therefore, by measuring the polarization state of the reflected beam, information about the thickness of the film layer can be derived.
Ellipsometers are quite useful for providing a relatively quick and non-invasive method of measuring film thickness. However, in most practical measurement systems, an error in the range of xc2x130% can be expected. As film thicknesses become increasingly shallower (i.e., less than 200 xc3x85), other techniques must be developed that exhibit a smaller range of error.
The need remaining in the art is addressed by the present invention, which relates to a measurement technique for ultra-thin oxides and, more particularly, to a technique that utilizes a monitor sample in conjunction with a device-to-be measured to provide for an accurate measurement of a deposited ultra-thin oxide film.
In accordance with the present invention, a reference sample (such as a silicon substrate) is processed to include (either by growth or deposition, for example) an oxide layer with a thickness (xe2x80x9ctxe2x80x9d) that is relatively easy for a conventional ellipsometer to measure. The reference sample and the actual production wafer(s) are then subjected to the same process for growing an ultra-thin oxide film, where in the case of the reference sample, the ultra-thin oxide will grow over the previously-formed oxide. A conventional ellipsometer can then be used again to measure the total thickness (xe2x80x9cTxe2x80x9d) on the reference sample, where the difference between these two values (Txe2x88x92t) is then defined as the thickness (xe2x80x9cxcex4xe2x80x9d) of the ultra-thin film oxide.
It is an aspect of the present invention that any suitable type of measurement tool can be used to determine the thickness of the oxide layers formed on the reference sample, as long as the measurement tool exhibits sufficient resolution to be able to distinguish between the thickness prior to and after growth of the thin-film oxide.
Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.